The recognition of soluble protein antigens by T-cells during the induction of immune responses appears to require that the antigen be "processed" and "presented" by accessory or antigen presenting cells (APC) the nature and origin of which have not been clearly established. These studies will use a model protein antigen, hemoglobin (Hb), which has a well characterized structure, to examine structural effects exerted on this antigen by APC required for its recognition by T-cells. Recent advances in the ability to clone antigen-reactive T-cells, the identification of APC that can be isolated as relatively homogenous populations (such as antigen-presenting tumor cell lines), and in the development of extremely sensitive analytical and separation techniques for complex mixtures of proteins and polypeptides (such as high performance liquid chromatography (HPLC)) make these studies feasible. Various cell populations, including macrophages, tumor-cells and B cells, will be studied for their ability to present Hb to murine Hb-reactive T-cell clones. The specificities of the T cells for various epitopes on Hb will be localized through the use of polypeptide fragments obtained by cleavage of Hb with cyanogen bromide. The effects that APC exert on the structure of Hb, particularly with regard to its proteolytic degradation, will be examined by attempts to detect (by HPLC) and identify peptide fragments of Hb generated after uptake of radiolabeled Hb by APC. If these studies are successful, a correlation may be made between the molecular structures of epitopes recognized by T cells and structural effects exerted on antigen by APC. Such findings will shed considerable light on the events that lead to the immune recognition of antigens by T cells and will be significant with regard to developing approaches for manipulating immune responses in both health and disease.